1. Technical Field
The present disclosure relates to measuring the size and number concentration of airborne particles. Specifically, this disclosure relates to rapid measurement of the particle size spectra and concentration in the ultrafine and sub-micrometer size ranges, with particle diameters from several nanometers up to about one micrometer.
2. Brief Discussion of Related Art
Most commonly, high-resolution particle sizing in the diameter range from several nanometers to many hundred nanometers is done by electrical mobility sizing. Electrical mobility methods are able to detect smaller particles than is possible by optical means, and for spherical particles the sizing is independent of unknown quantities such as particle refractive index or density.
The most common approach is differential mobility sizing, whereby a single mobility size is selected from an input polydisperse particle source by applying an appropriate voltage on a mobility drift tube. At a fixed voltage setting the particles exiting the drift tube all have the same mobility, and different sizes are selected by stepping through the voltages. At each selected size the particle concentration exiting the drift tube is measured using a condensation particle counter, which enlarges the selected particles through condensation to enable their detection by optical means. Size distributions are obtained by sequential measurements at different drift tube voltage settings, a process that can take several minutes (Liu and Pui, 1974). The scanning mobility particle spectrometer (SMPS) technique developed by Wang and Ragan (1989) has greatly improved the speed of mobility methods by rapidly scanning through the drift tube voltages, but the process still takes more than one minute to characterize an entire size distribution spectrum.
Another approach is electrometer-based mobility sizing systems, which measure multiple mobility sizes by placement of a series of electrometers along the collection electrode of the electrical mobility drift tube (Tammet et al, 1999, 2002). These instruments can measure complete size spectra at 1 Hz-10 Hz, but their size resolution is limited by the multiple charging associated with the use of a unipolar charger, and the lower concentration they can detect is limited by electrometer noise (Jeong and Evans, 2009). While they offer high time resolution, these electrometer based instruments have neither the sizing precision nor the sensitivity for atmospheric measurements, where particle concentrations aloft are often of the order of a 103 cm−3.
The differential mobility methods are capable of measurements at low particle concentrations, but are slow, requiring several minutes to complete each size distribution measurement. The electrometer-based methods are fast, but are unable to detect particles at low concentrations typical of the atmosphere, especially in background or remote locations. None of these currently available technologies provide both the time resolution and the sensitivity needed for precise, rapid measurements at typical atmospheric concentrations. This present disclosure describes a method to measure particle size spectra with the size resolution of the differential mobility methods, but with the time resolution of the electrometer methods. It enables the rapid measurement of ultrafine particle size distributions with time resolution of the order of seconds.